The present invention relates to a resistance unit for a motor speed control, and more particularly relates to a resistance unit for controlling a motor speed of driving a blower of an automobile air conditioner.
A prior art relating to a flat resistance for a blower control unit of an automobile air conditioner is set forth in U.S. Pat. No. 5,000,662. However, this type of flat resistance has disadvantages as follows:
Firstly, since a plurality of flat resistors are printed on one side of a resistance circuit board, a size of the board is relatively large. Therefore, the airflow generated by a blower is obstructed from the large size of the board, thereby decreasing the airflow in quantity and strength. The flat resistance is fastened to a bottom wall of a fan scroll in such a manner that the resistance circuit board projects into an air output passage within the fan scroll.
Secondly, a resistance circuit which is printed on one side of the board and having a predetermined pattern has therefore a limited amount of space between neighboring portions. Therefore, the neighboring portions of the thermally interfere with each other, causing a false operation of the resistance circuit and shortening a life of the same.
Thirdly, a plurality of lead frames are integrally formed with the board. Accordingly, there is an inconvenience in assembling the board with a socket. Furthermore, it is impossible to freely arrange the board onto a supporting frame of the socket. Moreover, a part of the lead frames of the board is simply inserted into the socket without any mounting means. Therefore, the board is easily separated from the socket by an impact.
Fourthly, a temperature fuse is interposed in a middle part of the pattern of the resistance circuit. Therefore, a by-product being generated from melting of the soldered portion of the temperature fuse causing damage to the resistance circuit.